This invention relates to thermoplastic adhesives that are activated by heat, especially as heat-seal layers or coatings on substrates for use in packaging. Various heat-activatable thermoplastic adhesives are known. Typically, the heat-seal adhesive layer or coating is locally positioned in contact with itself or with another receptive surface, locally heated under pressure to activate the adhesive material and cooled to effect bonding, e.g. to form a seam or seal.
In some instances, the heat-sealed structure is immediately or soon thereafter subjected to stresses tending to separate the structure while the adhesive is still hot and before it has developed its full ultimate strength at ordinary temperature. For example, in various form-fill-seal continuous packaging operations, a flexible strip of packaging film having a heat-seal coating or layer of adhesive material is continuously formed into a pouch with heat-sealed seams, the pouch is immediately filled with solid or liquid content and closed with another heat-seal, and the sequence is repeated at high rates. The pressure of the pouch contents and other stresses of the form-fill operation tend to open the freshly made hot seals. In other applications, a previously made heat-sealed structure may be re-heated as in a baking oven where the heat-activatable adhesive becomes softened by the heat and its strength is reduced.
It is known to use ethylene polymers such as polyethylene and copolymers of ethylene with various other monomers as heat-activatable adhesives, heat-seal layers and coatings, and the like. However, like many other heat-activatable adhesive materials, such ethylene polymers generally have less hot tack strength than is desired for some applications such as form-fill-seal packaging. Improved hot tack strength is still a desideratum of the art.
It would be desirable to have a heat-activatable thermoplastic adhesive material with improved strength at heat-seal temperature, i.e. such adhesive with improved hot-tack strength. Accordingly, it is an object of this invention to provide such adhesive with improved hot-tack strength. Another object is to provide improved means for effecting heat-seal bonding and structures having better hot bond strength. Other objects and advantages of the invention will become evident from the descriptions that follow.